The present invention is directed to a bipolar high-frequency transistor having a suitably doped and structured semiconductor chip of a doped silicon substrate and has base, collector and emitter contactings. The semiconductor chip is enclosed by a housing and the contacting thereof is connected to the respective base, collector and emitter terminals of the housing.
Discrete semiconductor components enable the optimization of individual performance features for specific circuit applications. By contrast thereto, what are usually restrictive compromises are necessary for process-oriented reasons given integrated circuits. Specialized discrete transistors are indispensable as a supplement of integrated circuits, specifically for modern communications and consumer electronics that increasingly opens up the frequency range above 1 GHz.
Si-npn transistors are available as active, discrete components for use in circuits up to approximately 4 GHz. Transistors on a GaAs substrate, particularly in the form of unipolar transistors or field effect transistors, exist for even higher frequencies. However, it is of considerable economic interest to expand the range of employment of discrete silicon transistors, particularly since silicon wafers can be processed significantly more inexpensively than gallium arsenide wafers.
Even though an extremely broad spectrum of discrete Si high-frequency transistors is available in the market place, they all have a common structure, that is, the silicon substrate of the active chip serves as the collector terminal. The silicon substrate, which, for example, is n.sup.+ -doped, has its collector contacting (metallization) on the underside of the chip connected to the collector terminal of the housing or is arranged thereon and secured thereto. Emitter and base terminals to the semiconductor are implemented in extremely fine geometries (.ltoreq.1 .mu.m) on the upper side of the chip. The emitter and base terminals, for example, are applied on the doped emitter and base regions at the surface of the semiconductor chip in, for example, the form of an interdigital metallization structure. These emitter and base contactings are then each respectively connected via a bond wire to the appertaining emitter or base terminals of the housing. Such Si chips are integrated, for example, in an SMD (surface mounted device) housing, whereby the housing terminal on which the Si chip is applied defines the collector terminal of the housing. Emitter and base are bonded via gold wires to the remaining terminals of the housing. Both the chip as well as the bond wires are protected by the housing.
The range of employment of such components is characterized by the limit frequency f.sub.T which extends up to 10 GHz in commercially obtainable Si high-frequency transistors. A further increase of the limit frequency requires an improvement of the chip technology, for example on the basis of finer structures or shallower doping profiles.
However, it is not only the performance capability of the silicon chip that is significant for a marketable high-frequency component. On the contrary, the housing is becoming more important for frequencies above 1 GHz. It is to be considered a high-frequency network that transforms the impedances at the housing at the outside to the chip. In particular, the inductance in the emitter branch which necessarily occur due to long bond wires and housing lengths limits the gain that can be externally produced.
Attempts at housing optimization, multiple emitter bonding, etc., have produced only marginal improvements. A comparison of two chips having 40 GHz and 8 GHz in the described, so-called common collector structure clearly shows that the far better properties of the 40 GHz chip are absorbed by the housing. Although this performance data is documented by suitable measuring methods "on wafer", it is nonetheless not possible according to the previous common collector structure to achieve the desired high-frequency properties at the completely housed transistors.